Andre Brandstötter

Wave propagation through disordered media without backscattering and intensity variations

A fundamental manifestation of wave scattering in a disordered medium is the highly complex intensity pattern the waves acquire due to multi-path interference. Here we show that these intensity variations can be entirely suppressed by adding disorder-specific gain and loss components to the medium. The resulting constant-intensity waves in such non-Hermitian scattering landscapes are free of any backscattering and feature perfect transmission through the disorder. An experimental demonstration of these unique wave states is envisioned based on spatially modulated pump beams that can flexibly control the gain and loss components in an active medium.

Focusing inside Disordered Media with the Generalized Wigner-Smith Operator

We introduce a wave front shaping protocol for focusing inside disordered media based on a generalization of the established Wigner-Smith time-delay operator. The key ingredient for our approach is the scattering (or transmission) matrix of the medium and its derivative with respect to the position of the target one aims to focus on. A specific experimental realization in the microwave regime is presented showing that the eigenstates of a corresponding operator are sorted by their focusing strength-ranging from strongly focusing on the designated target to completely bypassing it. Our protocol works without optimization or phase conjugation and we expect it to be particularly attractive for optical imaging in disordered media.

Constant-pressure sound waves in non-Hermitian disordered media

When waves impinge on a disordered material they are back-scattered and form a highly complex interference pattern. Suppressing any such distortions of a wave’s free propagation is a challenging task with many applications in a number of different disciplines. In a recent theoretical proposal, it was pointed out that both perfect transmission through disorder as well as a complete suppression of any variation in a wave’s intensity can be achieved by adding a continuous gain–loss distribution to the disorder. Here we propose a practical discretized version of this abstract concept and implement it in a realistic acoustic system. Our prototype consists of an acoustic waveguide containing several inclusions that scatter the incoming wave in a passive configuration and provide the gain or loss when being actively controlled. Our measurements on this non-Hermitian acoustic metamaterial demonstrate the creation of a reflectionless scattering wave state that features a unique form of discrete constant-amplitude pressure waves. In addition to demonstrating that gain–loss additions can turn localized systems into transparent ones, we expect our proof-of-principle demonstration to trigger interesting new developments, not only in sound engineering, but also in other related fields such as in non-Hermitian photonics.Perfect transmission of sound waves through a strongly disordered environment is demonstrated using a set of speakers that provide exactly the right input to counteract scattering by the disorder. These principles can also be applied to light.

Non-Hermitian focusing deep inside strongly disordered scattering media

The recent intense research in the newly founded area of PT-symmetric optics [1,2] has triggered a lot of attention in theoretical and experimental studies of non-Hermitian effects in photonics. In this context of composite photonic systems that contain gain and loss, we recently demonstrated for the first time the existence of so-called “constant-intensity waves” (or CI-waves) [3], which, in the presence of inhomogeneous gain-loss media, can evolve with constant intensity — a property known so far only for plane waves in free space. We show here that engineering the complex index of refraction on similar principles leads us also to other unexpected effects with novel functionalities. In particular, we construct specific index distributions that for one specific wavelength can cause strong focusing inside disordered media. Such a focusing is the result of the non-Hermitian interference of the modes of the medium and is impossible without loss and gain.

Constant amplitude sound waves in non-Hermitian metamaterials

We investigate the possibility for acoustic waves to propagate with a constant amplitude in disordered media. We find that this remarkable property is possible if one adds a tailored distribution of gain and loss on top of the disorder, making the medium non-Hermitian. We present the theory of constant-amplitude acoustic waves in both cases of continuous and discrete media, and provide an experimental demonstration in a metamaterial at audible frequencies.